Detecting Colored Fiber Contaminants in Wool Top Using Balanced Illumination

Abstract

Balanced illumination techniques have been used for many years to improve the detection efficiency for colored fiber contaminants in a white fiber mass, e.g., white wool top. The individual fibers in a mass of white wool are essentially transparent, and their images can be partially suppressed by a suitable choice of illumination to increase the contrast of the colored fibers. This paper uses geometrical optics to explain the partial suppression of images formed by transparent fibers under balanced illumination. The paper also shows that these images can be completely suppressed in the ideal situation, where transparent fibers with circular cross section are completely surrounded by a uniformly diffuse source of illumination. The performance of a balanced illumination system can be optimized by choosing illumination conditions that conform more closely to this ideal situation.

Detecting Colored Contaminants in Wool Using an Integrating Sphere

“…Light emitted from all points on the cavity walls is assumed to be uniformly diffuse and unaffected by the presence of a small viewing aperture in the roof of the cavity. Under these idealized conditions, the light intensity (luminance) at all points within the cavity is uniform and isotropic, and we can show that images formed by transparent fibers with circular cross section are completely suppressed [3]. This result can be generalized using thermodynamic arguments to show that transparent bodies of arbitrary shape become completely invisible under cavity illumination [ 1 ].…”

“…When the web is illuminated from the lower side only, i.e., from the side opposite the observer, the transparent fibers form images with dark boundaries; when the web is illuminated from the upper side only, the image boundaries appear bright against a dark background. An optimum balance of illumination from both opposed sources may be chosen so that the image boundaries for transparent fibers have approximately the same brightness as the background illumination, i.e., the white fiber mass becomes almost invisible [3]. Cavity illumination: Suppose that the upper and lower diffuse sources in Figure 2 are joined at their extremities to form a cavity with a luminous surface of arbitrary shape that completely encloses the fiber web, as shown in Figure 3.…”

“…This paper describes the design and testing of an illumination system based on an integrating sphere that maximizes the visual contrast of colored contaminants by suppressing the images formed by the white fiber mass. In theory [3], this illumination system should completely suppress images formed by transparent objects (e.g., white wool fibers); in practice it closely approaches this ideal situation.…”

Factors Influencing the Detection of Dark Fiber Contamination in Plain Weave Fabric'

“…A professional textile quality assurance inspector and three novices inspected the fabric samples initially from a position 9 feet (2.7 m) from the fabric and again from a distance of 3 feet (0.9 m) [2] while on a perch, using overhead fluorescent lighting and an opaque white background. The procedure was to have the fabric masked in such a way that only a full width 0.5 m long was visible at any stage.…”

“…The fabrics were then cut into small pieces 80 X 100 mm. Each piece was submerged in water in a pietrie dish and examined using the CSIRO dark fiber detector with the illumination balanced [2]. This was necessary to locate fibers that the inspectors might have overlooked.…”